An image capturing device (stereo camera) that includes two image sensors capable of capturing a stereoscopic image has been developed. When a stereoscopic image is captured, the image for the left eye is captured by the image sensor for the left eye, and the image for the right eye is captured by the image sensor for the right eye. Disparity, of which degree changes in accordance with the distance from the image capturing device to an object, occurs between the captured left and right images. The relationship between disparity and distance may be expressed by D=B×f/Z. Here, D denotes disparity, B denotes a baseline length, f denotes a focal length, and Z denotes the distance from the image capturing device to the object. In a case where an image is captured by two image sensors that are arranged in parallel, the disparity becomes large when the distance from the image capturing device to the object is short, becomes small when the distance is long, and becomes zero for an infinite distance.
A method has been developed that calculates distance information based on such a disparity occurred in the left and right images in order to calculate a length between two points in a captured image, and an example is a method that is disclosed by PTL 1 described below. As illustrated in FIG. 26, in PTL 1, two points Ps and Pe, that is, a measurement start position and a measurement end position, respectively, are specified (T1 and T2) in an image that is captured by the image capturing device, and the length between the two points Ps and Pe is calculated using three-dimensional positional data.
Further, in a technique disclosed by PTL 2, two optical systems both installed in a distal end of an endoscope device are used to obtain two images that have disparity. A user moves a cursor on a monitor and specifies a first point and then moves the cursor on the images. The length between the position of the specified first point and the position of a present cursor is measured. A continuously updated value is displayed, and a position and a length of the object are measured at all times.
These image capturing devices are also referred to as length measurement devices in a case where a main purpose is measurement of a length.